Let's add x kilograms of water. Solution concentration = solute mass fraction = solute mass/solution mass.

In 90kg original alcohol solution, the solute mass = 90 * 50% = 45kg, and the diluted solution mass is 90+X kg. If the solute mass is constant before and after, 45/(90+x) = 30%, and x = 60, so add 60kg of water.